Note on Cosmic Distances

In astronomy, distances are measured in units of light years,
where one light year is the distance that light travels in a
year10 trillion kilometers. For historical reasons having
to do with measuring distances to nearby stars, professional
astronomers use the unit of parsecs, with one parsec being equal
to 3.26 light years.

Astronomers compute the distance to remote galaxies (ones that
are more than about 20 million light years away) with Hubble's
law. According to Hubble's law, the universe is expanding in
such a way that distant galaxies are receding from one another
with a speed which is proportional to their distance. The
recession causes the radiation from a galaxy to shift to longer
wavelengthsthe red shift. From a measurement of the red
shift and the constant of proportionality, called Hubble's
constant, astronomers can determine the distance to a
galaxy.

One of the central problems of modern astronomy is to
accurately determine Hubble's constant, which is a measure of
the rate of expansion of the universe. At present it is known to
an accuracy of about 20 percent, so we usually modify distances
by saying "about 100 million light years," for example. We
assume throughout the Photo Album a value of the Hubble constant
that corresponds to a recession velocity of 600 kilometers per second for a source at a
distance of 30 million light years or
10 million parsecs (H0 = 60 km/s/Mpc).